1. Field of the Invention
The present invention generally relates to devices used in radiation therapy for the purpose of shielding from radiation exposure those areas unintended for treatment. More particularly, this invention relates to a radiation shield composed of a radiation-absorbing material dispersed within a thermoplastic matrix material that enables the shield to be readily molded and subsequently remolded for reuse a potentially infinite number of times, so as to yield a radiation shield that is highly effective and practical for use in radiation therapy.
2. Description of the Prior Art
In the treatment of malignancies using electron, gamma and x-ray radiation, modalities of therapy will vary according to tumor cell type, stage of lesion, degree of anaplasia, radioresistance of the malignancy and, finally, what healthy tissues will be altered or destroyed during the tumor irradiation process. Malignant tumors in the mouth, and head and neck regions generally require a mean therapeutic dosage of about 5000 to about 7000 centigray (cGy) in increments of 180 cGy per day, on a schedule of about five treatments per week. In some instances, about 4000 to about 5000 cGy may be initiated to the lymph nodes of the neck. In all cases, it is of concern to minimize damage to healthy tissue surrounding the cancerous tissue intended to be destroyed by therapy, in that cellular damage from radiation therapy, or radiotherapy, can range from minor cutaneous erythema to life-threatening osteoradenonecrosis.
To minimize injury to healthy tissue, radiation shields have been used for radiotherapy of highly localized regions of the body, including the thyroid, esophagus, laryngeal, tonsilar, sinus, basal and squamous, as well as for radiotherapy of melanoma near anatomic structures containing radio sensitive cells, such as the lip, eye, lacrimal ducts, ear, scalp and nasal mucosa. Additional uses of radiation shields include intra oral prosthesis for protecting the salivary glands, mucosa, tongue and dentosseous areas from injury such as xerostomia (dry mouth), loss of taste, mucositis, post radiation infection, radiation caries and osteoradenonecrosis. In particular, examples of oral tissue complications due to radiotherapy include mucositis from dosages in excess of greater than about 1000 cGy, erythatous mucositis from dosages in excess of about 2500 cGy, loss of taste from dosages in excess of about 3000 cGy, up to about 57% loss of salivary function from dosages in excess of about 1000 cGy, irreversible xerostomia from dosages in excess of about 4000 cGy, and mean beginning of osteoradenonecrosis from dosages in excess of about 5900 cGy.
Prior art shielding devices have generally entailed shields or stents formed as customized rigid prosthetic devices or from a moldable lead-filled clay. An example of an intra orally shield employed in the art utilizes Lipowitz metal containing toxic lead and cadmium, and entails a very time consuming impression, cast and fabrication process. While shielding devices formed from lead-filled clay have found use because they are remoldable, their use is limited by significant disadvantages and shortcomings. For example, lead-filled clay shields never exist in a rigid plastic state, and have relatively poor flow characteristics such that they do not readily capture fine anatomic detail in tumor fields. A consequence of their continual moldable character is that these devices have a tendency to change shape whenever handled, such as during removal from a patient, and must often be reshaped or remolded for each therapy session. Furthermore, it is difficult to measure the degree of radioresistance of these devices for use in critical radiosensitive healthy cellular organelles because they bend and distort when an attempt is made to gauge their thickness with a caliper.
In addition to the above, the lead particles of lead-filled clay shields tend to leach out of the clay and form a black oil residue that will leave marks on the patient's tissue. Such a tendency exacerbates the well known toxic nature of lead, rendering such devices particularly incompatible for intra oral use and at extra oral post-surgical wound sites, even for minimal exposures. Furthermore, lead-filled clay shielding devices are generally unable to absorb electron backscatter, which occurs with gamma and x-ray sources and pertains to the low energy electrons and positrons that are reflected and scattered by radiation shield materials. Finally, because lead-filled clay shields tend to become readily distorted, they cannot be used to precisely retarget the radiation field relative to the exact tumor location and with respect to healthy tissues before each radiation treatment.
An alternative to lead-filled clays is taught in U.S. Pat. No. 5,190,990 to Eichmiller, wherein particles of a nontoxic metal are dispersed in a moldable elastomeric base-catalyst thermosetting material. More specifically, Eichmiller teaches the use of various metal alloy spheroid particles in an addition-reaction polymerizable elastomeric precursor or resin, such as vinyl polysiloxane resin or silicone resin. The nontoxic metals suggested by Eichmiller offer greater safety to the patient than does a lead-filled clay, particularly for intra oral use. However, a significant drawback of the shield taught by Eichmiller is that the shield material is a thermoset, and therefore not remoldable, such that the shield's anatomic shape is irreversible. Such a limitation necessitates that the shield be trimmed with a knife or supplemented with additional polymer in order to ultimately achieve a suitable fit for radiotherapy, as well as provide the necessary thickness in order to ensure that adequate radioresistance is provided for the surrounding healthy tissue. Furthermore, Eichmiller's shield must generally be discarded after radiotherapy is completed for a particular patient, since it is highly unlikely that the shield will appropriately fit any other patient.
Because the thermosetting materials taught by Eichmiller are addition-reaction compositions, another complication is that a limited period of time is available during which the shield is moldable before the material permanently sets. Consequently, shields fabricated in accordance with Eichmiller are practically limited to being mixed and compounded in the clinic where treatment is to occur. In order to achieve a uniform distribution of metal particles within the thermosetting material, and therefore provide uniform radioresistance throughout the shield, the metal particles must be admixed while the thermosetting material is still highly workable. This constraint, and because the precured thermosetting materials may have a gel- like consistency, impedes the ability to achieve a uniform distribution of metal particles in the compound. As a result, the radioresistance of the shield may be reduced in localized regions, leading to exposure of healthy tissue to harmful levels of radiation. Another complication is that the time span for mixing the particles may be further limited if excess curing agent is used, or if the temperature of the compound or room is higher than that recommended for mixing. Consequently, the shield fabricator may not have adequate time to mix the compound and then make an accurate impression prior to the thermosetting material taking a permanent set.
Thus, it would be desirable to provide a radiation shielding device whose composition provides the necessary protection to healthy tissue during radiotherapy, is able to block backscatter electrons, and avoids the toxicity and permanent deformable nature of clay-based devices, yet is capable of being readily formed and remolded in order to enable the shield to be better tailored to the physical features of a particular patient, remoldable to adapt to changes in patient physiology and radiotherapy. Advantageously, such a shield could be readily reused with other patients if sterilized, heated and then remolded.